Abstract: Using a mouse model of hepatitis B virus (HBV), the goals of this proposal are to determine: 1) how group 3 innate lymphocytes (ILC3s) contribute to HBV disease pathogenesis and 2) whether age plays a role in ILC3 function during an HBV immune response. The first portion of this proposal focuses on studying how HBV pathogenesis is affected by ILC3-deficiency through monitoring serological markers of HBV clearance and persistence, measuring the strength and diversity of effector T cell responses, evaluating the extent and quality of B cell responses and assessing lymphoid organization in the liver. The role of specific ILC3-derived cytokines will also be investigated by blocking their signaling during HBV-induced immunity to determine which ILC3 effector molecules contribute to ILC3 function in the liver. The second part of this proposal focuses on quantifying and qualifying age-dependent differences in hepatic ILC3s in an effort to better understand how age directs human HBV pathogenesis. This will be accomplished by measuring the expression of key cytokines and surface molecules in liver ILC3s over time during the initiation of an immune response to HBV in adult and young mice, as well as investigating the impact that ILC3 stimulation has on HBV pathogenesis in an age-dependent manner. Finally, age-dependent patterns of ILC3s identified in the mouse model will be compared to adult and infant human liver biopsies to determine the relevance of these findings in humans. Information gathered from this proposal will represent significant progress towards understanding how immunity is orchestrated in the liver. It is clear that the liver employs unique strategies for determining self versus non-self due to the constant influx of non-self antigens from commensal organisms in the intestines, particularly at an early age when the GI tract is undergoing microbial colonization. Hepatitis B virus research shows that while the immune response in adults is often effective in controlling HBV infection, infants' and young children's immune responses in the liver are ineffective at clearing the virus, often leading to persistent infection. Thus, it is critical that we characterize age-dependent differences within the liver that direct effective versus ineffective immunity so that we can identify better therapeutic targets for the millions of patients living with chronic HBV. Defining the role of ILC3s in HBV pathogenesis is one step toward this goal and is essential for a comprehensive understanding of hepatic immunity. These studies will also garner insight on the greater role of ILC3s during infection and provide clues as to why these cells are most important in tissues that experience high microbial exposure.